Active telephone fine interface circuits, in which the tip and ring wires of a telephone subscriber fine are driven via an active fine drive circuit, are known. This invention is particularly applicable to, but is not in any way limited to, a form of active telephone line interface circuit certain features of which are described in Rosch et al. U.S. Pat. No. 5,274,702 issued Dec. 28, 1993 and entitled "Wideband Telephone Line Interface Circuit" and Rosenbaum et al. U.S. Pat. No. 5,323,461 issued Jun. 21, 1994 and entitled "Telephone Line Interface Circuit With Voltage Switching".
As described in these patents, in a telephone subscriber line interface circuit a line drive circuit includes two unity-gain d.c. amplifiers whose outputs are coupled to the tip and ring wires, respectively, of the telephone subscriber fine via a switching circuit and a sensing circuit. The line interface circuit also includes a controlled voltage generator and a control circuit. The sensing circuit serves to monitor a.c. and d.c. conditions on the line and includes a sensing transformer and series resistors which are generally referred to as feed resistors. The switching circuit serves, in association with software control via the control circuit, to interconnect the fine, line drive circuit, controlled voltage generator, and a battery in various configurations to provide for different operating states of the line interface circuit. These states include configurations in which relay contacts connect the outputs of the line drive circuit or the output of the controlled voltage generator selectively to the tip and ting wires of the line, and in which relay contacts and an electronic switch selectively connect the output of the controlled voltage generator or the battery as a supply voltage for the fine drive circuit.
It is desirable for the software control of the controlled voltage generator and the switching circuit to be such that switching operations to close relay contacts only take place at times when there are insignificant voltage differences across the relay contacts, so that high current flow on switching, and consequent damage to the relay contacts, is avoided. In practice, however, the desire for relatively rapid switching operations, and especially the inductive and generally unknown nature of the telephone subscriber line to which the line interface circuit is connected, result in conditions in which substantial voltage differences can exist across the closing relay contacts. In consequence, there can be a temporary high current flow which damages the relay contacts.
It is also well known to protect the line drive circuit in an active subscriber line interface circuit from high transient voltages due to lightning, a.c. surges, and inductive coupling. To this end, it is known to couple a diode bridge to points between the outputs of the line drive circuit and the feed resistors, for clamping positive transient voltages to ground and for clamping negative transient voltages to a breakdown voltage of a transient voltage suppressor which is connected between a negative supply voltage and ground. In order to provide sufficient power dissipation for negative transient voltages, it has been necessary to use several transient voltage suppression diodes (frequently referred to by the trade name TRANZORB or TRANSIL) in combination. This has the disadvantages of incurring a relatively high cost and requiring a relatively large physical space.
It would be desirable to be able to replace the several transient voltage suppression diodes by a single crowbar-type protection device, frequently referred to by the trade name TRISIL or SIDAC. However, such a device has the disadvantage that, if used in an active line interface circuit, the current supplied by the line drive circuit can be sufficient to maintain the protection device operative at and after the end of a negative transient voltage, resulting in a locked up state of the line interface circuit.
For the avoidance of any doubt, it is observed here that the TRANZORB and TRANSIL type of transient voltage suppression diodes are avalanche diodes which are specially designed to clamp excessive voltages (i.e. they provide a voltage clamping function) and to dissipate high transient power. In this specification they are referred to as transient voltage suppressors, and they are illustrated in the drawings in the same manner as zener diodes. In contrast, the TRISIL or SIDAC crowbar-type protection device has an electrical characteristic similar to that of a triac but with an internal triggering mechanism; it provides a very high impedance or open circuit for voltages up to a breakover voltage, and then provides a low impedance or short circuit as long as a holding current is maintained. In this specification this device is referred to as a crowbar-type protection device, and is illustrated in the drawings in the same manner as a triac but with a breakover bar instead of a trigger input.
An object of this invention is to provide an improved telephone line interface circuit in which the above disadvantages of the prior art are reduced or avoided.